The plates are always shifting. The Enriquillo-Plantain Garden Fault (the fault system that caused the Haiti quake) was shifting about 20 cm/yr before the quake. A segment of the fault at a depth of 11 km near Port au Prince ruptured; and suddenly moved 480 cm and caused a massive quake.

This particular fault is a strike-slip fault. The North American and Caribbean plates are moving past one another. The incredible force of the plate movement is mostly offset by the incredible force of friction between the two plates. Occasionally sections of faults will rupture and the opposing fault blocks will locally move very rapidly for a brief period of time.